Variable valve controls via camshaft adjustment of an internal combustion engine are described in, for example, the essays “Dance of the Valves” and “Variable Valve Control” in the magazine “mot Profi Spezial Variable Ventilsteuerung” [mot Profi Spezial Variable Valve Control], no. 3/2002, Vereinigte Motor-Verlage, Stuttgart.
The camshaft adjustment allows a mechanical variation of the valve trigger times at the intake and discharge channel of a cylinder chamber. The adjustment of the camshaft in the advance or retard direction that is required for this purpose is implemented with the aid of a rotary-vane adjuster, for example, or similar means, via the oil pressure. The oil pressure in the individual chambers is controlled by solenoid valves, which are addressed via a pulse-width modulated signal sent from the control device. In the process, the camshaft must attain the setpoint value with sufficient accuracy within a prescribed period of time. If this is not the case, a malposition is present. Malpositions during variable camshaft adjustment lead to worsening emissions, among others, due to a no longer optimal combustion; they may even result in combustion misses. Furthermore, the driver will complain about a lack of torque or poor output and losses in smooth running such as jerking, which is the result of faulty camshaft positions and the associated charge differences between the cylinder groups.
The diagnosis of the camshaft control is currently performed by checking the angle of adjustment of the camshaft. For this purpose, the amount difference between the setpoint value and the instantaneous value is compared to applicable threshold values. If the threshold values are exceeded or not attained for an applicable period of time, a fault of the camshaft adjustment unit will be set. An adjustment fault of the camshaft is set whenever the instantaneous angular position is outside a permissible tolerance range after the debouncing time has elapsed. A camshaft diagnosis according to the related art is able to diagnose only a general adjustment error of the camshaft.